古巨基's medley has never charted. 陈奕迅's K歌之王(No.1 in 2001) is the nearest as a medley to hit the chart, as it uses 用心良苦, 爱如潮水 as bridge, lyrics is taken from many songs, but itself is an original composition.

Mashed up are 2 different songs that can be merged into one. See Numb/Encore, Boulevard of broken dreams/Wonderwall. so, Rainie's song is not really a mashed up. It is a medley. anyway, it is still the first proper medley to chart.

Regarding mashed up, this song did not chart, but probably was the 1st mashed up I have heard:

It secretly uses Macarena as the music!

by the way, Vivian Chow is so gorgeous in her 1st hit on the chart. Pity she never reached No.1.